There's definitely opposition from native americans, it's not a movement that lefty white people just made up one day when they couldn't find anything else to be pissed about.
A majority of Native Americans polled by a UC Berkeley researcher object to caricatures of their culture
news.berkeley.edu
Basically they found if a person is Liz-Warren-tier ""Native American"", then they're far more likely to say they don't care about this stuff. But the more engaged a person was with tribal business and identity, the more likely they were to say they found the team names and mascots offensive.
I don't believe there's any offense intended in these names or mascots or gestures. In modern times, anyway. It's just sports fandom. I also think people who are fans of the Indians or the Braves or the (former) Redskins are probably more likely to seek out knowledge around native american tribes and history than the average joe, which can be a positive thing.
So I get why there's resistance from fans to change any of it, because it's like being forced to agree that your passion in following the team all those years was somehow causing harm.
But if I'm a fan of one of these teams, arguing to keep the name when people from the depicted group are telling me it's offensive isn't really a battle I'm interested in having. I don't need 51% of a population to claim offense to change my mind. I'd rather just change it and move on. Which is probably what the ownership is thinking.